A, an intracellular sharp microelectrode recording from a CA1 pyramidal neuron in the presence of NBQX, AP-5 and CGP-55845. Superimposed Vm responses (upper traces) evoked by HFS during bridge mode current injection (±0.2 nA, 2 s; lower traces) reveal a dramatic fall in the input resistance of the cell and show that the response in Vm evoked at resting membrane potential is nearly identical to EGABA throughout the HFS period. The large increase in membrane conductance decays only after the HFS period, thereby allowing spiking during the prolonged depolarization. The dashed line demonstrates measurement of the rate of change of the response in mV s−1 during HFS in the absence of injected current that was used to obtain estimates of the initial rate of rise in [Cl−]i (see Methods for details). B, the HFS-induced pronounced increase in membrane conductance is blocked by 10 μm bicuculline (BMI). Traces in A and B are from the same continuous recording. C, HFS-induced Vm responses in a CA1 pyramidal neuron in the presence of NBQX, AP-5 and CGP-55845, and after a subsequent application of 1 mm furosemide. Note that the rate of intracellular Cl− accumulation depends not only on the rate of change in EGABA but also on the level of EGABA (see Methods). The timing of the HFS pulses is shown with a horizontal bar and by the stimulation artefacts in A, B and C. Stimulus artefacts have been truncated from the original traces.